PROJECT SUMMARY It is well accepted that activation of the medial olivocochlear (MOC) efferent system can result in a reduction of cochlear gain in humans and laboratory animals. In humans, efferent-induced cochlear gain reduction is measured by measuring changes in the magnitude of otoacoustic emissions. Because the reported changes have been small and the perceptual effects of gain reduction due to stimuli presumed to elicit efferent activation have been elusive, the general view has been that MOC efferents play a minor (if any) role in everyday auditory processing. This proposal should overturn that view. The proposal has been motivated by large robust effects of MOC efferent activation observed in our preliminary experiments that took advantage of recent improvements in otoacoustic-emission measurement techniques and in our understanding of the mechanisms of the generation of stimulus frequency otoacoustic emissions (SFOAEs). The large physiological effects will be combined with novel psychophysical measures obtained in the same listeners. In Aim 1 we will pursue the phenomenon and functional relevance of a recent and highly unexpected finding in our lab of large and very long lasting (on the order of seconds) efferent-based effects on SFOAEs that seem to be exclusive to high frequencies. We will also explore sizeable perceptual unmasking of temporal amplitude modulation of a tone embedded in noise observed in our preliminary studies, the time course of which exhibits frequency dependence similar to that of the long-lasting efferent effects on SFOAEs. This previously unreported unmasking effect may have particular functional importance, as the processing of amplitude modulation in noise has recently been shown to be crucial for the understanding of speech. Aim 2 combines novel implementations of stimulus-frequency otoacoustic emissions (SFOAEs) with psychoacoustic paradigms to provide rigorous tests of the link between cochlear tuning and SFOAE delay, and to assess the effects of MOC system on physiological and behavioral tuning. The outcomes should provide a definitive assessment of the currently controversial claim that human cochlear tuning may be considerably sharper than that of typical laboratory animals. In Aim 3 we will implement a novel approach to investigating the modulating effect of attention on MOC efferent effects on cochlear responses by selectively manipulating perceptual load and cognitive load of behavioral auditory and visual tasks. The outcomes of the research should result in a refined more complete view of the functional role of MOC efferents in human auditory processing. By showing sizeable effects of efferent activation that benefit perception in normal-hearing listeners, the proposal will underscore the importance of restoring these benefits in hearing-impaired listeners, including cochlear-implant users. The proposed experiments will provide important new data on the effects of efferent control of cochlear gain that will aid such efforts and lead to new algorithms to recreate the efferent-driven modulation of cochlear gain and tuning in hearing-assisting devices.